1. Field of the Invention
The present invention relates to an image pickup apparatus which is capable of normal photography (picture taking) in air, and a photography in water.
2. Description of the Related Art
Even in an optical system in which a distortion is corrected favorably in a taking optical system on the land, when an underwater housing having a plane parallel plate in an entrance window is mounted, and an attempt is made to take a picture under water, a pin-cushion distortion occurs. This is because a refractive index of air is approximately 1.00 and a refractive index of water is approximately 1.33306.
A graph shown in FIG. 12 is a graph which shows a relationship between a half-image angle of incidence in water, and an amount of distortion occurred, when light subjected to corrected distortion is incident on a taking optical system via the entrance window in the form of a plane parallel plate, in the water. A vertical axis shows the half-image angle of incidence (degrees) and a horizontal axis shows the amount of distortion (%) which occurs. It is revealed that as the image angle becomes large, the pin-cushion distortion becomes remarkable.
On the other hand, when the entrance window toward (in) the underwater housing side is made to be dome shaped, and when each main light ray is let to be incident substantially vertically with respect to the entrance window, a taking with distortion is suppressed. However, in this case, the entrance window and a water medium together act as a concave lens, and a field curvature occurs. Moreover, a manufacturing cost of the dome-shaped entrance window becomes high.
Moreover, when a chromatic aberration of magnification is corrected favorably on land, in underwater photography through the plane parallel plate, due to an effect of dispersion of water, (Abbe's number is approximately 62), a chromatic aberration of magnification of a short wavelength occurs in a positive direction (a direction going away from a center of a picture plane). On the other hand, the chromatic aberration of magnification of a long wavelength occurs in a negative direction (a direction approaching the center of the picture plane), and a color spreading is susceptible to occur.
Inventions which compensate an aberration change at the time of underwater photography are disclosed in Japanese Patent Application Laid-open Publication Nos. Hei 7-159689 and 2004-252219. An invention in which a lens member and a diffraction filter are added so as to correct the chromatic aberration of magnification or the distortion which occurs during the underwater photography has been disclosed in Japanese Patent Application Laid-open Publication No. Hei 7-159689. Moreover, an idea of correcting the chromatic aberration by using a diffraction surface in the entrance window of the underwater housing has been disclosed in Japanese Patent Application Laid-open Publication No. 2004-252219.